1. Field of the Invention
The present invention relates to an information processing system that includes a host processing device, an external storage device, and a control device intervening between the host processing device and the external storage device, and especially to a technology to control data look-ahead from the external storage device to a cache memory within the control device in random access input/output request processing from the host processing device.
2. Related Background Art
The common technology to control data look-ahead from an external storage device to a cache memory inside a control device predicts future access pattern based on history information on patterns of access from a host processing device and controls look-ahead. In making the prediction, accumulating access pattern history information is the key.
Technology regarding a look-ahead control through access pattern prediction is known. According to the technology, cache hit/miss information based on a plurality of access requests recently accepted from a host processing device is managed and stored as statistical information for each physical region that was accessed; when one of the regions is subsequently accessed again, the statistical information is referred to and the possibility of cache hit if data are loaded to the cache is determined; if the possibility is high, data in the region, as well as neighboring regions, are loaded to the cache memory.
A technology to optimize the amount of data to be loaded to a cache memory is known. According to the technology, a hit rate is calculated based on the number of cache hits and misses in the past for a block to be accessed, and the amount of data to be loaded to the cache memory is dynamically varied according to the level of the hit rate.
When performing a processing in which random accesses to a database are concentrated within a certain period of time, conventional methods that determine a loading mode to a cache based on access pattern history information have poor sensitivity due to the random nature of the random accesses and cannot offer the optimum loading mode; they cannot realize a rapid improvement in the cache hit rate, input/output requests from a host processing device result in cache misses and therefore in accesses to physical devices (generally HDDs, which are magnetic disk devices), and they rely on the performance (seek time+time for a disk to make one revolution+transfer time) of one HDD for responsiveness.